In recent years, the demand for communications has increased rapidly as a result of the spread of the Internet and so forth. High capacity networks have accordingly been completed at high speed by using optical fiber. Further, in order to establish high-capacity communications, an optical multiplexing technology that transmits a plurality of channels' worth of optical pulse signals together via one optical fiber transmission line has been investigated.
As optical multiplexing technology, optical time division multiplexing (OTDM), wavelength division multiplexing (WDM) and optical code division multiplexing (OCDM) have been intensively researched. Among these technologies, OCDM has the merit of flexibility on the operation side, that is, of having no restrictions on the time axis allocated one bit at a time of the optical pulse signals that are transmitted and received in OTDM and WDM and so forth. Further, OCDM has the merit that a plurality of channels can be established in the same time slot on the time axis or a plurality of communication channels can also be established with the same wavelength on the wavelength axis.
Subsequently, the expression ‘optical pulse signal’ signifies an optical pulse train reflecting a binary digital signal. That is, an optical pulse train reflecting a binary digital signal in correspondence with the existence and nonexistence of optical pulses constituting the optical pulse train on a time axis with respect to an optical pulse train in which optical pulses stand in a row at regular fixed intervals (time interval corresponding to the reciprocal of the frequency corresponding to the bit rate) is an optical pulse signal.
OCDM is a communication method that extracts signals by means of pattern matching by allocating codes (patterns) that are different for each channel. That is, OCDM is an optical multiplexing technology that encodes an optical pulse signal by means of an optical code that is different for each communication channel on the transmission side and which restores the original optical pulse signal by performing decoding by using the same optical codes on the reception side as on the transmission side.
Because only the optical pulse signals whose codes correspond are extracted and processed as effective signals during decoding, an optical pulse signal that consists of light rendered by combining the same wavelengths or a plurality of wavelengths can be allocated to a plurality of communication channels. Further, because a passive light element such as a Fiber Bragg Grating (FBG) can be used as the phase control means of the optical encoder to perform the phase control required for code processing, it is possible to deal with a higher communication rate without the encoding processing being subject to electrical restrictions. Further, suppose that a plurality of channels can be multiplexed at the same time and same wavelength and large-capacity data communications are possible. In comparison with OTDM and WDM and so forth, the focus is on being able to rapidly increase the communication capacity.
As OCDM encoding means, an optical phase code system that uses the phase of light as code is known. More specifically, a Superstructured Fiber Bragg Grating (SSFBG) is used as the encoder and decoder (See Non-patent documents 1 and 2, or Patent document 1, for example).
The operating principles in a case where an optical pulse time spreader comprising phase control means formed by using an SSFBG encoder is used as an encoder and decoder will now be described with reference to FIGS. 1(A) to 1(E). FIG. 1(A) shows the time waveform of an input optical pulse. FIG. 1(E) serves to illustrate an aspect in which an encoded optical pulse train that has been encoded by an encoder is decoded by a decoder.
The input optical pulse shown in FIG. 1(A) is encoded as a result of being input to an encoder 10 from an optical fiber 12 via an optical circulator 14. The input optical pulse then passes through the optical fiber 18 via the optical circulator 14 once again before being input to a decoder 20 via an optical circulator 22. Further, an autocorrelation waveform is generated as a result of decoding by a decoder 20 and the autocorrelation waveform passes through an optical fiber 26 via the optical circulator 22.
The encoder 10 and decoder 20 shown in FIG. 1(E) are an SSFBG constituted by arranging four unit Fiber Bragg Gratings (FBG) in the waveguide direction of the optical fiber. Here, as an example, the functions of the encoder 10 and decoder 20 will be described by using a four-bit optical code (0, 0, 1, 0). Here, the number of items in the numerical sequence consisting of ‘0’s and ‘1’s that provides the optical code is also called the codelength. In this example, the codelength is 4. Further, the numerical sequence providing the optical code is called a code string and each item ‘0’ and ‘1’ of the code string is also known as a chip. Further, the values 0 and 1 are also called code values.
The unit FBGs 10a, 10b, 10c, and 10d constituting the encoder 10 correspond with a first chip ‘0’ of the abovementioned optical codes, a second chip ‘0’, a third chip ‘1’, and a fourth chip ‘0’ respectively. The determination of whether the code value is 0 or 1 is the phase relationship of the Bragg reflected light that is reflected by adjacent FBG units. That is, because the first chip and second chip have an equal code value 0, the phase of the Bragg reflected light reflected by unit FBG 10a corresponding with the first chip and the phase of the Bragg reflected light reflected by unit FBG 10b corresponding with the second chip are equal. Further, because the code value of the second chip is 0 and the code value of the third chip is 1, the two chips have mutually different values. Therefore, the difference between the phase of the Bragg reflected light reflected by unit FBG 10b corresponding with the second chip and the phase of the Bragg reflected light reflected by unit FBG 10c corresponding with the third chip is n. Likewise, because the code value of the third chip is 1 and the code value of the fourth chip is 0, the two chips have mutually different values. Therefore, the difference between the phase of the Bragg reflected light reflected by unit FBG 10c corresponding with the third chip and the phase of the Bragg reflected light reflected by unit FBG 10d corresponding with the fourth chip is π.
Thus, because the phases of the Bragg reflected light from the unit FBGs are changed, the specified optical code is also known as ‘optical phase code’.
A process in which an autocorrelation waveform is formed as a result of an optical pulse being encoded by an encoder and converted to an encoded optical pulse train and the encoded optical pulse train being decoded by a decoder will be described next. When the single optical pulse shown in FIG. 1(A) is input from the optical fiber 12 to the encoder 10 via the optical circulator 14 and optical fiber 16, Bragg reflected light from the unit FBGs 10a, 10b, 10c, and 10d is generated. Therefore, suppose that the Bragg reflected light from the unit FBGs 10a, 10b, 10c, and 10d is a, b, c, and d. That is, the single optical pulse shown in FIG. 1(A) is converted into an encoded optical pulse train as a result of time spreading of the Bragg reflected light a, b, c, and d.
When the Bragg reflected light a, b, c, and d is represented on a time axis, an optical pulse train resulting from arrangement at specified intervals that depend on the method of arranging the unit FBGs 10a, 10b, 10c, and 10d on the time axis through division into four optical pulses is constituted as shown in FIG. 1(B). Therefore, an encoded optical pulse train is an optical pulse train that is produced as a result of time-spreading an optical pulse that is input to the encoder as a plurality of optical pulses on a time axis.
FIG. 1(B) shows an encoded optical pulse train that passes through the optical fiber 18 with respect to the time axis. In FIG. 1(B), for the purpose of a quick representation of the encoded optical pulse train, the optical pulses are shown displaced in the vertical axis direction.
The Bragg reflected light of unit FBG 10a is the optical pulse denoted by a in FIG. 1(B). Likewise, the Bragg reflected light of FBG 10b, FBG 10c, and FBG 10d are optical pulses denoted by b, c, d respectively in FIG. 1(B). The optical pulse denoted by a is an optical pulse that is reflected by the unit FBG 10a closest to the input end of the encoder 10 and is therefore in the most temporally advanced position. The optical pulses denoted by b, c, and d are each Bragg reflected light from the FBG 10b, FBG 10c, and FBG 10d respectively. Further, the FBG 10b, FBG 10c, and FBG 10d stand in a line in a row from the input end of the encoder 10 and, therefore, the optical pulses denoted by b, c, and d stand in a line in the order b, c, d after the optical pulse denoted by a as shown by FIG. 1(B). In the subsequent description, the optical pulses corresponding with the Bragg reflected light a, Bragg reflected light b, Bragg reflected light c, and Bragg reflected light d respectively are also represented as the optical pulse a, optical pulse b, optical pulse c, and optical pulse d. Further, the optical pulse a, optical pulse b, optical pulse c, and optical pulse d are also each called chip pulses.
The relationship between the phases of the Bragg reflected light a, b, c, and d that constitute the encoded optical pulse train is as follows as mentioned earlier. The phase of the Bragg reflected light a and the phase of the Bragg reflected light b are equal. The difference between the phase of the Bragg reflected light b and the phase of the Bragg reflected light c is π. The difference between the phase of the Bragg reflected light c and the phase of the Bragg reflected light d is π. That is, when the phase of the Bragg reflected light a is taken as the reference, the phases of the Bragg reflected light a, Bragg reflected light b, and Bragg reflected light d are equal and the phase of the Bragg reflected light c differs by π from the phases of the Bragg reflected light a, Bragg reflected light b, and Bragg reflected light d.
Therefore, in FIG. 1(B), the optical pulses corresponding with the Bragg reflected light a, the Bragg reflected light b and Bragg reflected light d are denoted by solid lines and the optical pulse corresponding with the Bragg reflected light c is denoted by a dotted line. That is, in order to distinguish the relationship between the phases of the respective Bragg reflected light, solid lines and dotted lines are used to represent the corresponding optical pulses. The phases of the optical pulses denoted by a solid line are in a mutually equal relationship and the phases of optical pulses denoted by dotted lines are in a mutually equal relationship. Further, the phases of the optical pulses denoted by a solid line and the optical pulses denoted by a dotted line differ by π from one another.
An encoded optical pulse train is input to the decoder 20 via the optical circulator 22 after passing through the optical fiber 18. Although the decoder 20 has the same structure as the encoder 10, the input end and output end are reversed. That is, the unit FBGs 20a, 20b, 20c, and 20d stand in a line in order starting from the input end of the decoder 20 but the unit FBG 20a and unit FBG 10d correspond. Further, a unit FBG 20b, unit FBG 20c and unit FBG 20d likewise correspond with the unit FBG 10c, unit FBG 10b, and unit FBG 10a respectively.
In the encoded optical pulse train that is input to the decoder 20, the optical pulse a constituting the encoded optical pulse train is first Bragg-reflected by the unit FBGs 20a, 20b, 20c, and 20d. This aspect will be described with reference to FIG. 1(C). In FIG. 1(C), the horizontal axis is the time axis. Further, the relationship before and after a time is illustrated by expediently assigning 1 to 7, where smaller numerical values denote increasingly early times.
FIG. 1(C) shows an encoded optical pulse train with respect to the time axis in the same way as FIG. 1B. When the encoded optical pulse train is input to the decoder 20, the encoded optical pulse train is first Bragg-reflected by unit FBG 20a. The reflected light that is Bragg-reflected by unit FBG 20a is shown as ‘Bragg reflected light a’. Likewise, the reflected light that is Bragg-reflected by the unit FBG 20b, unit FBG 20c, and unit FBG 20d is shown as the Bragg reflected light b′, c′, and d′ respectively.
The optical pulses a, b, c and d constituting the encoded optical pulse train are Bragg-reflected by unit FBG 20a and stand in a line on the time axis of the string denoted by a′ in FIG. 1(C). The optical pulse a that is Bragg-reflected by unit FBG 20a is an optical pulse that has a peak in a certain position that is denoted by 1 on the time axis. The optical pulse b that is Bragg-reflected by unit FBG 20a is an optical pulse with a peak in a certain position that is denoted by 2 on the time axis. Likewise, the optical pulse c and optical pulse d are optical pulses with a peak in a certain position denoted by 3 and 4 respectively on the time axis.
The optical pulses a, b, c, and d that constitute the encoded optical pulse train are also Bragg-reflected by unit FBG 20b and stand in a line on the time axis of the string denoted by b′ in FIG. 1(C). The Bragg-reflected reflected light b′ that is reflected by unit FBG 20b has a phase that is shifted by π in comparison with the phases of the Bragg-reflected light a′, c′ and d′. Therefore, the string of optical pulses that stand in a line on the time axis of the string denoted by a′ and the string of optical pulses that stand in a line on the time axis of the string denoted by b′ have phases that are all shifted by π.
As a result, whereas a string of optical pulses that stand in a line in the order 1 to 4 on the time axis denoted by a′ stand in a line in the order of a solid line, solid line, dotted line, and solid line, a string of optical pulses that stand in a line in the order 2 to 5 on the time axis denoted by b′ stand in a line in the order of a dotted line, dotted line, solid line, and dotted line. The displacement on the time axis of the optical pulse train denoted by a′ and the optical pulse train denoted by b′ is because, among the optical pulses constituting the encoded optical pulse train, the optical pulse a is input to the decoder 20 before the optical pulse b.
Likewise, the optical pulses a, b, c, and d that constitute the encoded optical pulse train are also Bragg-reflected by the unit FBG 20c and unit FBG 20d and the optical pulses stand in a line on the time axis of the strings denoted by c′ and d′ respectively in FIG. 1(C). The Bragg-reflected light c′ and d′ reflected by the unit FBG 20c and unit FBG 20d have phases that are equal in comparison with the Bragg-reflected light a′. Therefore, in FIG. 1(C), the optical pulse train denoted by c′ and the optical pulse train denoted by d′ stand in a line on the time axis. The optical pulses related to the Bragg-reflected light a′, c′, and d′ are shifted in parallel on the time axis but the mutual phase relationship between the optical pulses related to the Bragg-reflected light is the same.
FIG. 1(D) shows the autocorrelation waveform of the input optical pulses that are decoded by the decoder 20. The horizontal axis is the time axis and corresponds to the illustration shown in FIG. 1(C). The autocorrelation waveform is obtained by the sum of the Bragg-reflected light a′, b′, c′, and d′ from the respective unit FBGs of the decoder and, therefore, all the Bragg-reflected light a′, b′, c′ and d′ shown in FIG. 1(C) is brought together. Because the optical pulses related to the Bragg-reflected light a′, b′, c′ and d′ are all added together with the same phase at the time shown as 4 on the time axis of FIG. 1(C), a maximum peak is formed. Further, because two optical pulses denoted by a dotted line and one optical pulse denoted by a solid line are added together at the time shown as 3 on the time axis of FIG. 1(C), one optical pulse's worth of peaks whose phases differ by π are formed for the maximum peak at the time shown as 4. Further, because two optical pulses denoted by a solid line and one optical pulse denoted by a dotted line are added together at the time shown as 1 on the time axis of FIG. 1(C), one optical pulse's worth of peaks whose phases are equal are formed for the maximum peak at the time shown as 4.
As described hereinabove, the optical pulses are encoded by the encoder 10 to produce an encoded optical pulse train and the encoded optical pulse train is decoded by the decoder 20 to generate an autocorrelation waveform. In the example taken here, an optical code (0,0,1,0) of four bits (codelength 4) is used but the description above is equally valid even in cases where optical code is not used.
The schematic structure of conventional phase control means will now be described with reference to FIGS. 2(A) and 2(B). FIG. 2(A) is a schematic cross-sectional view of the phase control means. The phase control means has a structure in which an SSFBG 30 is fixed to a core 34 of an optical fiber 36 comprising the core 34 and cladding 32. The SSFBG 30 is constituted such that 15 unit FBGs are arranged in series in the waveguide direction of the core 34 constituting the optical waveguide of the optical fiber 36.
When the optical phase code which is set for the phase control means of the conventional optical pulse time spreader shown in FIG. 2(A) is written as a 15-bit code string, the result is (0,0,0,1,1,1,1,0,1,0,1,1,0,0,1). Further, the relationship of correspondence between the abovementioned optical code and the unit FBGs arranged in series in the core 34 is as follows. That is, the unit FBGs, which are arranged in a direction extending from the left end to the right end of the SSFBG 30 shown in FIG. 2(A) and the chips, which are arranged in a direction extending from the left end to the right end of (0,0,0,1,1,1,1,0,1,0,1,1,0,0,1) that represents the optical codes of the unit FBGs noted as the abovementioned 15-bit code string correspond with one another one-on-one.
FIG. 2(B) schematically shows the refractive index modulation structure of the SSFBG 30 shown in FIG. 2(A). The horizontal axis is a position coordinate in the longitudinal direction of the optical fiber 36 forming the SSFBG 30. The vertical axis represents the refractive index modulation structure of the optical fiber 36 and the difference between the maximum and minimum of the effective refractive index of the optical fiber 36 is represented as Δn. Further, in FIG. 2(B), the refractive index modulation structure of the optical fiber 36 is drawn partially enlarged.
The refractive index modulation cycle is Λ. Therefore, the Bragg reflection wavelength λ is given by λ=2NeffΛ. Here, Neff is the effective refractive index of the optical fiber 36. In the subsequent description, the effective refractive index is also simply called the refractive index for the sake of simplification.
In FIG. 2(A), when the phases of the Bragg-reflected light of adjacent unit FBGs differ by π, the intervals between adjacent unit FBGs are shown shaded black. Further, when the phases of the Bragg reflected light of adjacent unit FBGs are equal, an optical modulation structure in which the intervals between the unit FBGs are continuous is shown. On the other hand, in FIG. 2(B), when the phases of the Bragg-reflected light of adjacent unit FBGs differ by π, black triangles are shown added to the intervals of the two unit FBGs.
When the phases of the Bragg reflected light of adjacent unit FBGs are equal, the refractive index modulation structure of the two unit FBGs is a continuous cycle structure. On the other hand, when the phases of the Bragg reflected light of adjacent unit FBGs differ by π, the refractive index modulation structure of the two unit FBGs have a shift of only π (jump in the pie phase) inserted at the boundary between the two unit FBGs.
Table 1 shows the relationship between the optical phase code (0, 0, 0, 1, 1, 1, 1, 0, 1, 0, 1, 1, 0, 0, 1) and the phase difference of the Bragg reflected light of adjacent unit FBGs for implementing the optical code. At the top of Table 1, the code values of the optical phase code established for the conventional phase control means shown in FIG. 2(A) are shown lined up in a row as code. Further, the phase difference of the Bragg reflected light of adjacent unit FBGs is shown as the phase shift amount in the bottom level of Table 1. The unit FBGs arranged extending from the left end to the right end of the SSFBG 30 shown in FIG. 2(A) and the chips arranged extending from the left end to the right in brackets representing the optical phase code (0, 0, 0, 1, 1, 1, 1, 0, 1, 0, 1, 1, 0, 0, 1) correspond one for one.
TABLE 1
The geometric interval between adjacent unit FBGs in which the phase shift amount is π is converted to a phase value to become π/2 due to light traveling there and back between adjacent unit FBGs. Generally, when the interval between adjacent unit FBGs for which the phase shift amount is π is converted to a phase value, the interval is given by πN+(π/2) with N as an integer. That is, the phase difference of the Bragg reflected light from adjacent unit FBGs for which the phase shift amount is π is given by 2πN+π. Further, the geometric interval between adjacent unit FBGs for which the phase shift amount is 0 is converted to a phase value and given by πN, and the phase difference of the Bragg reflected light from the two unit FBGs is given by 2πN.
Further, subsequently, when the phase shift amount is written, general notation such as πN+(π/2) is sometimes omitted and also written simply as π/2.
[Non-Patent Document 1]
Akihiko Nishiki, Hisashi Iwamura, Hideyuki Kobayashi, Satoko Kutsuzawa, Saeko Oshiba ‘Development of OCDM phase encoder using SSFBG’ Technical Report of IEICE. OFT2002-66, (2002-11),
[Non-Patent Document 2]
Hideyuki Sotobayashi, ‘Optical code division multiplexing network’, Applied Physics, Volume 71, 7. (2002) pages 853 to 859,
[Patent Document 1]
U.S. Pat. No. 6,628,864,